A two-dimensional scanner may be formed by using two single-axis scanners, scanning around two orthogonal axes. Most two-dimensional laser beam scanners are for lasers with wavelength in the range from 350 nm to 1500 nm. Mirrors with metallic or dielectric coatings in this wavelength range can have relatively high reflectivity (&gt;98%) for input laser beam in either s or p polarization and for a large range of incident angle (&gt;10 degrees). High throughput can be easily achieved for scanners in this wavelength range and is thus not a special issue in the design of these scanners.
Two-dimensional scanners with good and consistent throughput are much more difficult to manufacture for lasers with wavelength in the deep UV range from 150 nm to 250 nm. In this wavelength range, usually only dielectric-coated mirrors can be used. Due to a limited selection of coating materials, the reflectivity of a dielectric coating in this wavelength range is much sensitive to the polarization and the incident angle of an input beam. For a deep-UV mirror specified for 45 degree incidence, the reflectivity for s polarization is typically 98%, while for p polarization is typically 92%. The reflectivity drops significantly from its optimum value if the incident angle is a few degrees off from an optimum angle of incidence. This optimum angle of incidence changes sensitively with the laser wavelength. In addition, the optimum angle of incidence for optimum reflectivity may shift from its specified angle, may vary from coating to coating runs, and may change with humidity and the age of the mirrors.
An X-Y scanner for a deep-UV laser beam has two of these dielectric mirrors and the transmission loss is usually higher than 10%. Most scanners are designed for large angle scanning and for lasers with random polarization. A typical design is to make the out-going beam from the scanner perpendicular to the input beam. Two examples are commercial x-y scanners from General Scanning, Inc. and Cambridge Technology, Inc. For those scanners, significant improvement on their throughput for deep-UV beam is difficult without improving the mirrors available.
In some applications, linearly-polarized laser beams are used and only small angle scanning is required. An example of these applications is photorefractive surgery with a deep-UV laser beam from a solid-state laser source. It is highly desirable for these applications to manufacture two-dimensional scanners with improved throughput by employing commonly available mirrors.